I think you can get it at health food/whole food stores sometimes.  You can make it yourself by very, very slowly heating grocery store butter until the oil separates from the solids.  Then you can carefully separate the oil from the solids, put the oil part in a leak proof container and use it.
It does stay fresh longer than butter; but I have not had salted butter go rancid on several two week trips, and longer if spring or fall camping in cooler temps.